In order to facilitate the provision of services to user terminals, a mobile network such as a 3G network will often require the establishment of a secure communication channel or “security association” between client terminals (i.e. mobile terminals) and the network-based service nodes which provide the services. The Generic Bootstrapping Architecture (GBA) is discussed in the 3GPP Technical Specification TS 33.220 and provides a mechanism whereby a client terminal (UE) can be authenticated to a Network Authentication Function (the service node), and secure session keys obtained for use between the client terminal and the Network Authentication Function. The simple network model for this architecture is illustrated in FIG. 1. This mechanism bootstraps upon the known Authentication and Key Agreement (AKA) procedure [3GPP TS 33.102] which allows a client terminal to be authenticated to a Bootstrapping Server Function (BSF) of the client's home network on the basis of a secret K which is shared between the USIM of the client terminal and the Home Subscriber System (HSS) of the subscriber's home network. The AKA procedure further establishes session keys from which keys are derived that are afterwards applied between the client terminal and a Network Application Function (NAF). When a client terminal and NAF wish to obtain session keys from the BSF, the NAF sends a transaction identifier to the BSF, the transaction identifier containing an index which the BSF uses to identify the client terminal and appropriate keys which it forwards to the NAF.
According to the GBA mechanism, a UE initiates the key generation process by sending a request containing a user identity to the BSF. The request also contains the identity of the NAF. The BSF retrieves an authentication vector from the Home Subscriber System (HSS), each authentication vector consisting of a random number RAND, an expected response XRES, a cipher key CK, an integrity key IK and an authentication token AUTN. The BSF generates key material KS by concatenating CK and IK contained within the authentication vector. The BSF generates a key identifier B-TID in the format of a NAI by base64 encoding the RAND value and combining the encoded value with the BSF server name, i.e. as                base64encode(RAND)@BSF_servers_domain_name.        
The BSF retains the key KS in association with the transaction identifier B-TID and the NAF identity. The B-TID and AUTN are sent by the BSF to the UE, the USIM of the client terminal verifying the value AUTN using the shared secret K and returning a digest of the expected result XRES to the BSF. The USIM also generates the key material KS using the secret K and the value RAND (recovered from the B-TID).
Following completion of this procedure, the UE communicates to the NAF, the received B-TID. The NAF and the BSF are authenticated to one another, and the NAF sends to the BSF the received B-TID together with its own identity. The BSF uses the B-TID and the identity of the NAF to locate the correct key KS, and uses KS to generate a NAF key. Other information such as the NAF identity is also used in the generation of the NAF key. The generated NAF key is returned to the NAF. The UE is similarly able to generate the NAF key using the key KS that it has already generated.
After the GBA mechanism has been run for the first time, subsequent requests to establish a security association between the UE and the same or a different NAF may use the already established key material KS, providing that key has not expired. However, this will still require that the UE initiate a request for establishment of a security association by sending its B-TID to the NAF.